Method and device for cleaning filters for dust-laden waste gases

ABSTRACT

The invention relates to a method for cleaning filters for dust-laden waste gases, comprising several filter elements ( 2 ) which are arranged vertically in a filter housing ( 1 ) and which have an upper open end ( 16 ) and a lower closed end ( 15 ) with at least one feed line ( 9 ) for the dust-laden waste gases and at least one discharge line ( 11 ) for the cleaned waste gases, in addition to a device for injecting surges of compressed air into the open end ( 16 ) of the filter elements ( 2 ). In order to reduce pressure fluctuations in the filter and to provide efficient cleaning with pulsations of compressed air in the low pressure range i.e. from approximately 0.8 to 3 bars, the filter is divided into several filter modules ( 1 ) respectively comprising at least one filter element ( 2 ). At least two filter modules ( 1 ) are arranged in a filter housing ( 10 ) or a filter chamber. The filter modules ( 1 ) are cleaned alternately and each filter module ( 1 ) is associated with a discharge line ( 11 ) with a shut-off element ( 3 ) for the clean gas. A control device ( 19 ) is provided for the shut-off elements ( 3 ), whereby when the filter modules ( 1 ) which are to be cleaned are changed, the shut-off element ( 3 ) of the cleaned filter modules ( 1 ) is open and the shut off element ( 3 ) is closed in a diametrically opposed manner for the filter module ( 1 ) to be cleaned. As a result, any pressure fluctuations in the filter are minimised.

[0001] The invention relates to a method and device for dedustingfilters for dust-laden waste gases, including several verticallyarranged filter elements each having an upper, open end and a lower,closed end, wherein the waste gases flow through the filter elementsfrom outside and the cleaned waste gases are discharged through the openends of the same, and wherein for dedusting compressed-air blasts areblown into the open ends of the filter elements to be dedusted.

[0002] Filters for dust-laden waste gases are used where impurities suchas as chips, shreds, fibers or dusts are to be separated from aconveying air and disposed of or recycled. Examples of industrialbranches where lots of dust occur include the wood-processing industry,fiber industry, paper industry or petroleum industry. Impurities are,for instance, sucked off the location of a processing machine, and thepolluted waste gases, which are referred to as crude gases, are suppliedto the filter. A filter, as a rule, is comprised of several filterelements, which may be designed as filter hoses, filter cartridges,filter bags or even filter plates. The crude gas to be cleaned flowsthrough the filter medium from outside towards inside, whereby theimpurities and, in particular, dust and aerosol particles are separatedon the outer side of the filter element. The purified gas stream leavesthe filter element through the upper, open end into a so-calledclean-gas chamber and through appropriate ducts reaches consecutivelyarranged plant components or escapes into the atmosphere.

[0003] In the course of time, the deposit formed on the outer walls ofthe filter elements, which is called filter cake, becomes thicker andthicker, for which reason the filter must be periodically dedusted inorder to guarantee the continuous operation of the same. To this end,air is blown in through the open end of the filter element in order toseparate the impurities adhering to its outer side. In doing so,dedusting is effected either by a continuing scavenging-air flow or by ashort and vigorous compressed-air blast. The advantages of thescavenging-air method reside in a lower pressure of the dedusting airflow and the low mechanical stresses thus exerted on the filtermaterial. Yet, it is disadvantageous that large amounts of scavengingair are required to efficiently remove the filter cake. The presence ofmovable parts within the dedusting mechanism involves further drawbacks.Moreover, the dedusting effect will be particularly insufficient withcritical or tacky dusts as well as high hose resistances. The highvolume flows required as well as additional measures such as, forinstance, the heating of the scavenging air for hot-gas filtration bringabout further economic disadvantages.

[0004] By contrast, the compressed-air method offers the advantages thatno moving parts are required within the dedusting mechanism and that themethod functions well with critical dusts and high hose resistances,leading to optimum dedusting results. Further advantages would includelow energy demands, low volume flows of compressed air and no preheatingof the dedusting air necessary for hot-gas filtration in order to avoidlower deviations of the dew point. The disadvantages involved incompressed-air methods include high pressures prevailing in the pressurereservoirs as well as thus caused high mechanical loads exerted on thefilter medium and hence reduced service lives of the filter elements. Inaddition, the throwing off of dust particles from a filter element islikely to cause suction through the adjacent filter element, for whichreason no sustained removal of the filter cake will be achieved. Adevice for dedusting filter hoses or filter cartridges by blowingcompressed air into an injector nozzle is, for instance, described in AT377 711 B.

[0005] In the dedusting of filters of the initially defined kind, it isdistinguished between what is called online dedusting, which is effectedwithout any shutdown of the plant, and so-called offline dedusting,during which a short-term interruption of the circulation is effected.In online dedusting air is blown into individual filter elements, orcompressed-air blasts are fed to the latter, during the operation of thefilter plant, while the operation of the remaining filter elements iskept going.

[0006] Since part of the energy is lost through the upper, open end ofthe filter element during the dedusting of a filter element such as,e.g., filter hose, methods enabling higher dedusting rates have beendeveloped by at least partially covering during dedusting the open endsof the filter elements to be dedusted, so that the filter element to bededusted will be more or less free of pressure. Such a device for thesuccessive dedusting of hose filters is, for instance, described in AT407 840 B.

[0007] Another method and device for cleaning a dust separator isdescribed in AT 405 615 B, wherein, in order to ensure a good cleaningeffect, the filter elements to be dedusted are blocked on the clean-gasside and swept around by a directed crude-gas flow during dedusting suchthat the throwing-off of the filter cake from the filter element to bededusted will be assisted by said crude-gas flow. That method involvesthe drawback that during dedusting a certain extent of polluted crudegas will sweep around the filter element to be dedusted. Consequently,the applied pressure of the compressed air used for dedusting must beraised, or a poorer dedusting effect will have to be taken into account.The crude-gas flow also causes the filter cake thrown off the dedustedfilter element to be conveyed to other filter elements, where it isagain sucked in rather than caused to drop into the dust collectionfunnel by the shortest way possible.

[0008] A further disadvantage of known dedusting methods consists inthat pressure fluctuations will occur in the filter, or in the overallplant, due to the switching on and off of the filter elements before andafter dedusting. Heavy pressure deviations that may affect the wholesystem are particularly caused during the switching on of a dedustedfilter element, or a filter chamber comprised of several filter elementsjust dedusted. The control of the fan or the like, which is responsiblefor the discharging of the clean gas, can thus, for instance, fall outof step by sudden pressure changes. Due to the inertia of the fan,pressure deviations cannot be immediately compensated for and hence maylead to vibrations. On the other hand, changes in the pressuredifference between the filter plant and its surroundings may also occuron the site of origin of the dust-laden waste gases, for instance in aprocessing machine or mill or the like, whereby an air flow in theopposite direction of the suction plant may even be caused.

[0009] DE 23 45 722 A1 describes a gas filter in which a dedustingnozzle extends over the entire length of the mouths of the filterelements and is attached to a movably arranged nozzle carriage. Thatarrangement does not comprise several filter modules arranged in acommon housing and does not disclose any measures to minimize pressuredeviations during the dedusting of filter modules.

[0010] DE 27 09 204 A1 describes a method for cleaning flowing gases anda corresponding device in which compressed air is blown into the filterelements in at least two separate pulses, the first pulse serving toloosen the filter cake and the subsequent pulse serving to blast thesame off the filter hose.

[0011] DE 27 25 438 A1 discloses a method and device for the blow-backproof compressed-air blast cleaning of filter elements, in which anelastically designed membrane valve is arranged on the mouth of thefilter body. By hermetically sealing off the open filter hose duringdedusting, it is ensured that the compressed air will become fullyeffective during dedusting, yet pressure deviations in the filter arenot minimized.

[0012] Finally, DE 28 31 167 A1 shows a filter comprising filterelements arranged within a chamber, wherein during the dedusting of achamber the latter is locked both on the crude-gas side and on theclean-gas side, thus enhancing the dedusting intensity. The closure ofboth the crude-gas-side flap and the clean-gas-side flap calls for theuse of an overflow valve during dedusting in order to relieve theoverpressure prevailing in the closed chamber. A high pressure withinthe housing would reduce the dedusting effect of a compressed-air blastor of the scavenging air. That arrangement is characterized by highconstruction expenditures.

[0013] It is the object of the present invention to provide a method anddevice for dedusting filters for dust-laden waste gases, by whichpressure deviations in the system can be minimized to the largest extentpossible while, at the same time, rendering feasible the sustainedremoval of the filter cake, the application of as low a pressure aspossible for the compressed air used for dedusting, and the consumptionof a small compressed-air volume. The expenses involved in the methodand device according to the invention are to be kept as low as possibleso as to also minimize production and installation costs.

[0014] The object according to the invention in method terms is achievedin that at least one filter element is each arranged in a filter moduleand that at least two filter modules are each arranged in a housing orfilter chamber, whereby the filter modules are alternately dedusted withthe filter module to be dedusted being set in a substantially flow-freestate during dedusting by interrupting, during dedusting, the clean-gasdischarge duct of the filter module to be dedusted and effecting theconnection of the dedusted filter module to, and the disconnection ofthe filter module to be dedusted from, the gas flow in a diametricallyopposed manner at a change-over of the filter modules to be dedusted,thus enabling the minimization of any possible pressure deviations inthe filter. By subdividing the filter into filter modules and bringingthe filter module during dedusting into a substantially flow-free state,offline dedusting of the filter module can be ensured without requiringthe usual chambered mode of construction of filters, which involves alarge number of supply ducts, discharge ducts and partition walls. Here,the complete separation of the filter modules to be dedusted from thecrude-gas flow is not sought, because this would imply elevatedconstruction costs as with the chambered mode of construction. Moreover,it is advantageous that the crude-gas supply duct remains open duringthe dedusting of a filter module, so that the pressure wave propagatingduring dedusting can propagate via the crude-gas duct rather thancausing an excessive pressure to build up in the filter module or filterhousing, respectively. Due to the diametrically opposed connection ofthe dedusted filter module to the gas flow, and the disconnection of thefilter module to be dedusted from the gas flow, as provided by theinvention, pressure deviations in the filter as well as in the overallplant are minimized and the overall pressure situation is stabilized.Consequently, low pressures may be applied for the dedusting of thefilter elements while nevertheless ensuring the effective removal of thefilter cake. By the respective filter module being in a substantiallyflow-free state during dedusting, dedusting and the throwing-off of thefilter cake will not be affected and the sedimentation of the thrown-offfilter cake will not be hampered, either. The substantially flow-freestate of the filter module is obtained during dedusting by theinterruption of the clean-gas discharge of the filter module to bededusted, which may, for instance, be effected by conventional shutoffmeans such as various types of valves. Because of the use ofcompressed-air blasts in the low-pressure range, the energy required forsuch compressed air blasts is minimized and also the volume of thecompressed air consumed is kept low. Furthermore, the modular mode ofconstruction involves fewer technical expenditures and hence fewerproduction costs. Another consequence of the stable pressure conditionsis the relatively uniform amount of dust occurring, which has positiveeffects on the subsequent treatment of the same, since, for instance,the discharging means such as, e.g., screw conveyors or conveyingdevices, will not be differently stressed and hence will not requiredifferent activation. The order of filter module dedusting can be chosenat will. The filter elements within a filter module can be provided withcompressed-air blasts either simultaneously or consecutively, said orderbeing likewise arbitrarily feasible with a large number of filterelements.

[0015] For dedusting, compressed-air blasts at a reservoir or receiverpressure of 0.5 to 3 bar, preferably 0.8 to 2 bar, are used. With thistype of dedusting, the compressed-air volume per dedusting pulse ispreferably 0.5 to 4 liters per m² of filter area. By comparison,conventional online operation is run at reservoir pressures ranging from3 to 10 bar. In doing so, the consumption of compressed air for eachpulse is 4 to 20 liters per m² of filter area. The respective pressuresof the compressed-air blasts depend on the respective lengths of thefilter elements. The indicated values are typical of filter hoses havinglengths of more than 4 m. By reservoir or receiver pressure, thepressure prevailing in the pressure tank or in the compressed-air supplyduct rather than the pressure prevailing at the nozzle above the openend of the filter element is understood, which, as a rule, issubstantially lower than the reservoir or receiver pressure. In additionto reduced energy costs, the saving of filter elements and the reductionof pressure fluctuations in the filter and overall system are feasiblewith lower pressure values.

[0016] In order to further reduce the energy on the one hand and keepthe load on the filter elements as low as possible on the other hand,only one compressed-air blast is used for each filter element of thefilter module per dedusting cycle for the dedusting of each filtermodule.

[0017] In order to achieve the optimum dedusting effect, it is providedthat the compressed-air blasts are directed in the direction of thefilter elements. This can be ensured by certain structural measures suchas, e.g., compressed air nozzles provided above the open end of thefilter element or injectors arranged in the open end of the filterelement, or similar.

[0018] In order to minimize the necessary volume of compressed air, itis provided according to a further characteristic feature of theinvention that the compressed-air blasts have a duration of less than300 ms. By duration, the electric opening time of the respective valvefor the compressed-air nozzle is to be understood rather than themechanical opening time of the valve, which usually deviates slightlyfrom the former.

[0019] If the filter module to be dedusted is left in the substantiallyflow-free state for a certain period of time after dedusting, thesedimentation of the thrown-off filter cake will be enhanced andsupported, since a new suction of the impurities descending towards thedust collection funnel will be prevented, if the impurities are givenenough time to descend into the dust collection funnel.

[0020] The object according to the invention is also achieved by adevice for dedusting filters for dust-laden waste gases, includingseveral filter elements vertically arranged in a filter housing and eachhaving an upper, open end and a lower, closed end, at least one supplyduct for the dust-laden waste gases and at least one discharge duct forthe cleaned waste gases as well as a means for blowing compressed-airblasts into the open ends of the filter elements, wherein the filter issubdivided into several filter modules each comprising at least onefilter element and at least two filter modules are arranged in a filterhousing or filter chamber, said filter modules being alternatelydedusted, and wherein each of said filter modules is each associatedwith a clean-gas discharge duct in which a shutoff means is provided forthe interruption of the clean-gas discharge duct, and a device forcontrolling the shutoff means is further provided so as to effect in adiametrically opposed manner, during a change-over of the filter modulesto be dedusted, the opening of the shutoff means of the already dedustedfilter module and the closure of the shutoff means of the filter moduleto be dedusted, thus enabling the minimization of possible pressuredeviations in the filter. If the clean-gas flow is interrupted, nocrude-gas flow will sweep around the associated filter module, thusenabling a lower pressure to be applied for the dedusting of the filterelement without any deterioration of the dedusting effect. It is onlythis measure that allows the compressed-air-blast-based dedusting methodto be used in the low-pressure range, i.e., between 0.5 and 3 bar whilesafeguarding high separation rates at the same time. Due to the factthat the filter element to be dedusted is in a substantially flow-freestate, the throwing off of the filter cake from the filter element,particularly from the outer wall of the filter hose, is not impeded andthe filter cake is able to drop down into the dust collection funnelsubstantially by the shortest way without being conveyed by thecrude-gas flow to adjacent filter elements and depositing there anew.The advantages are that dedusting need not be effected against thepressure of the crude-gas flow and can, thus, take place at a lowerpressure and a reduced volume flow. This enables the sustained removalof the dust cake. As opposed to known filter chambers, constructionexpenditure are lowered by the present invention due to the subdivisioninto filter modules. In addition, the diametrically opposed opening ofthe shutoff means of the already dedusted filter module and the closureof the shutoff means of the filter module to be dedusted ensure that theresulting pressure deviations will be minimized both in the filter andin the overall system.

[0021] According to a variant embodiment, each the filter modules iscomprised of a chamber forming the clean-gas space and including aconnection to the clean-gas discharge duct as well as a connection tothe at least one filter element. Such an embodiment is made up of butfew structural components and can be readily and quickly incorporatedand installed in existing filter housings. As opposed to known filterchambers, costs can, thus, be markedly reduced.

[0022] The advantages will be further enhanced in that partitionelements are arranged between the or some filter modules located withinthe filter housing. It is thereby rendered more readily feasible toreach the flow-free state of the filter module to be dedusted andadditionally prevent the adherence of the filter cake to the filterelements of adjacent filter modules arranged in a filter housing.

[0023] In order to ensure the guidance of the compressed-air blasts intothe open ends of the filter elements, at least one nozzle oriented inthe direction of the filter element is arranged above the open end ofeach filter element according to a further characteristic feature of theinvention.

[0024] Even better conditions will be reached in that two nozzlesoriented in the direction of the filter element are arrangedeccentrically above the open end of each filter element.

[0025] Further improvements of the cleaning effect will be obtained ifan injector is arranged in the open end of each filter element. Theenhanced flow conditions, in turn, allow for the application oflow-pressure compressed-air blasts, which will save both energy and thefilter elements.

[0026] In an advantageous manner, the at least one nozzle is arrangedabove the open end of each filter element at a distance from the entryopening of the injector.

[0027] If, in accordance with a further characteristic feature of theinvention, the at least one waste-gas supply duct is arranged below thelower end of the filter elements of the filter modules, a substantiallyflow-free state of the filter module to be dedusted will be obtained byshutting off the clean-gas duct without requiring several crude-gassupply ducts for each filter module.

[0028] The advantages of the method according to the invention andexemplary devices for carrying out said method will be explained in moredetail with reference to the accompanying drawings. Therein:

[0029]FIG. 1 is a schematic view of an embodiment of a filter module;

[0030]FIG. 2 is a sectional illustration of an embodiment of a filter;

[0031]FIG. 3 is a sectional illustration of a filter variant;

[0032]FIG. 4 is a sectional illustration of another filter variant; and

[0033]FIG. 5 depicts time-dependent diagrams of the shutoff means of thefilter modules of a filter to illustrate timing.

[0034]FIG. 1 shows a filter module 1, in which at least one filterelement 2 comprised of a filter hose is arranged. As a rule, a row offilter elements 2, or even several rows of filter elements 2, arearranged in a filter module 1. Each one of said filter elements 2 isvertically arranged and comprises a closed, lower end 15 as well as anopen, upper end 16. Each one of said filter elements 2 is suspended inan opening of a plate 17 that separates the crude-gas space 8 from theclean-gas space 7, the crude-gas flow introduced into the filter housing1 through a supply duct 9 flowing through each of said filter elementsfrom outside towards inside. The impurities contained in the crude-gasflow remain adhered to the outer walls of the filter elements 2, forminga filter cake that will grow over time. The cleaned crude gas reachesthe clean-gas space 7 via the upper, open end 16 of the filter element2, and from there flows to consecutively provided plant parts, or intothe atmosphere, via a suitable discharge duct 11. A propulsion jet tube4 is arranged above each row of filter elements 2 to remove the filtercake adhering to the filter elements 2, said propulsion jet tube 4 beingconnected with the respective pressure tank 4 b via a shutoff valve 4 a.This pressure tank is the reservoir or reception vessel. The propulsionjet tube 4 comprises an opening formed by one or several nozzles 5 aboveeach open end 16 of each filter element 2. During the dedustingprocedure, a compressed-air jet expands from this nozzle 5 above theclean-gas space 7 into an optionally provided injector 6, into theinterior of the filter element 2. The compressed air emerging from thenozzle 5 creates a pressure wave along the longitudinal axis of thefilter elements 2 in the direction of the closed end 15. Within theinjector 6, a mixture of primary air and secondary air is created bypulse exchanges. At the same time, this mixture is imparted a pressureincrease within the injector 6. At the emergence from the injector 6, ofthe mixture comprised of primary and secondary air, a pressure wave isformed along the longitudinal axis of the filter elements 2 in thedirection of the closed end 15 of the filter elements 2. At first, apressure backup is created there, because the dedusting air impinges onthe closed ends 15 of the filter elements 2, which are frequentlyprotected by metal caps, and is rebound from the same. The returningpressure wave inflates the filter elements 2. Due to the suddenlyoccurring pressure change and the reversal of the flow direction, thefilter elements 2 plus filter cake are accelerated towards outside witha sudden retardation taking place as the maximum expansion of the filterelement 2 is reached, whereby the filter cake is separated from theouter walls of the filter elements 2 and pops off the same. Thethrown-off filter cake 10 drops into a dust collection funnel 14downwardly connected to the filter housing 10 surrounding the filtermodule 1 and there is carried off, for instance, by means of a screwconveyor. If several rows of filter elements 2 are arranged within afilter module 1, these can be powered by compressed-air pulses eithersimultaneously or at short time delays of, for instance, 3 seconds. Indoing so, it is advantageous to not only power one row of filterelements 2 after the other, but to mix the order of rows of filterelements 2 so as to assist sedimentation.

[0035]FIG. 2 is a sectional side view of an embodiment of the invention,wherein two filter modules 1 each comprising at least one filter element2 are arranged in a filter housing 10. On account of the mode ofconstruction of the filter modules 1 described in FIG. 1, it is feasibleto use existing filter housings 10 by simply and rapidly inserting thefilter modules 1 into the same. Via a supply duct 9, the crude gas isfed into the crude-gas space 8 of the filter. The clean-gas space 7follows upon the upper, open ends 16 of the filter elements 2 and issubdivided by a partition wall 18. From each part of the clean-gas space7, a discharge duct 11 for clean air leads to further plant componentsor into the atmosphere. A shutoff means 3 is provided in each of thedischarge ducts 11 to block the clean-gas discharge duct. The shutoffmeans 3 of the discharge ducts 11 are connected to a control device 19.That device 19, which in most cases is comprised of a computer, controlsand regulates the opening and closing of the shutoff means 3 of allfilter modules 1. In accordance with the invention, the filter module 1to be dedusted is switched into a flow-free state at least duringdedusting by closing the shutoff means 3 of the respective dischargeduct 11. This causes the flow from the crude-gas space 8 to theclean-gas side to be interrupted and the associated filter module 1 toassume its flow-free state. In order to ensure the optimum sedimentationof the removed filter cake from the filter elements 2 of the dedustedfilter module 1, the shutoff means 3 may remain closed for a certainperiod of time upon completion of dedusting, thus facilitating thedescent of the filter cake down to the dust collection funnel 14. Afterdedusting of the filter elements 2 of the respective filter module 1 hasbeen completed, another filter module 1 is being dedusted, while thealready dedusted filter module 1 is again available for filtering, thuspermitting the continuous operation of the filter. In accordance withthe invention, the opening of the shutoff means 3 of the alreadydedusted filter module 1 and the closing of the shutoff means 3 of thefilter module 1 to be dedusted are controlled in a diametrically opposedmanner so as to minimize possible pressure deviations in the filter. Thecontrol implemented in the device 19 serves to ensure that alwaysexactly one filter module for dedusting will be switched off the gasflow such that the system will not be adversely affected by pressuredeviations. For an explanation of the timing, it is referred to thetime-dependent diagrams depicted in FIG. 5 as well as the pertinentdescription.

[0036]FIG. 3 illustrates a variant of a filter according to theinvention, in which a partition wall 12 is provided in the crude-gasspace 8 between the filter modules 1, or rows of filter elements 2 ofthe filter module 1, respectively. This partition wall 12 prevents thefilter cake from being sucked in by the adjacent row of filter elements2 and facilitates the sedimentation of the removed filter cake in thedirection towards the dust collection funnel 14. Because of the singlesupply duct 9 provided for the crude gas, the construction expenditureare still low as compared to conventional filter designs in the form ofindividual chambers, every filter chamber having to include all thenecessary structural components. With a larger number of filter modules1 arranged in a filter housing 10, partition walls need not be providedbetween all of the filter modules 1, but only between some of the filtermodules 1.

[0037]FIG. 4 is a sectional illustration of another variant of a filter,wherein filter modules 1 according to FIG. 1 are installed in a filterhousing 10 which is subdivided into three chambers. Three filter modules1 are used in each filter chamber. Each of the filter modules 1 may becomprised of a filter element 2 or rows of filter elements 2. Thedischarge ducts 11 for the clean air are combined to a common duct.Apart from that, no cumbersome mounting operations are necessary. Theshutoff means 3 provided in the discharge ducts 11 of each filter module1 are connected to a control and regulation device 19, which may, forinstance, be realized in the form of a computer. In the exampleillustrated, the shutoff means 3 of the second filter module 1, which isdenoted by M2, is shut off so that the filter elements 2 of the filtermodule M2 are in the flow-free state. Via the propulsion jet tube 4provided above the filter elements 2 of the filter module M2, acompressed-air pulse is sent into the filter elements 2, and the shutoffmeans 3 is kept in the closed position for a certain period of time suchthat the filter cake thrown off the filter element 2 can descend intothe dust collection funnel 14 and be carried off the same. After this,the shutoff means 3 of the filter module M3 is closed and, at the sametime, the shutoff means 3 of the filter module M2 is opened in adiametrically opposed manner. The order of dedusting of the filtermodules 1 need not necessarily be one after the other, but may bedistributed over all of the filter modules 1 of the filter according toa predefined scheme. The timing of the dedusting procedure according toFIG. 4 is explained in more detail in FIG. 5.

[0038]FIG. 5 shows time-dependent diagrams for the control of theshutoff means 3 of n filter modules 1 provided in a filter. By closingthe shutoff means 3, the filter module M1 is switched off the crude-gasflow at a certain instant and brought into a substantially flow-freestate. The closing of the shutoff means 3 of the filter module M1 iseffected during an interval Δt, as illustrated in FIG. 5 in theuppermost time-dependent diagram. The shutoff means may, for instance,be realized by disc valves or butterfly valves, which are connected witha control device 19. It should be noted that the process of opening andclosing the shutoff means 3 need not necessarily be linear asillustrated, but, as a rule, will rather deviate from linearity. As soonas the filter module M1 has been set into a substantially flow-freestate, a compressed-air pulse is sent into the filter element(s) 2 ofthe filter module M1. The compressed-air pulse for the filter elementsof the filter module M1 is denoted by D1 in the lowermost time-dependentdiagram of FIG. 5. After the filter module M1 has been dedusted, it isstill left in the flow-free state for a certain period of time so as topromote the sedimentation of the thrown-off filter cake. After this, thefilter module M1 is reset into the crude-gas flow by appropriatelyopening the associated shutoff means, thus annulling the substantiallyflow-free state. Such opening of the shutoff means again involves acertain time interval Δt. During the opening of the shutoff means of thefilter module M1, the shutoff means of another filter module M2 isclosed in a diametrically opposed manner so as to provide a glidingtransition of the filter modules and reduce any pressure deviationsoccurring in the system. It goes without saying that certain tolerancesin the timing of the shutoff means are permissible, anyway. As soon asthe filter module M2 has reached its flow-free state, a compressed-airpulse D2 is sent into the filter elements of the filter module M2, andupon expiration of a certain phase aimed to assist sedimentation theshutoff means of the filter modules M2 is re-opened. Simultaneously withthe opening of the shutoff means of the filter module M2, the shutoffmeans of a further filter module M3 is closed, whereupon said filtermodule M3 is dedusted. This process is continued until the last filtermodule Mn has been dedusted, whereupon the process is started anew, forinstance, with filter module M1. As already pointed out above, the orderof the dedusting of filter modules M1 to Mn is not critical. Changingconditions such as, for instance, elevated amounts of dust-laden wastegases can be responded to by reducing the time intervals between theshutoffs of the individual filter modules. By controlling the shutoffmeans of the individual filter modules M1 to Mn, as provided by theinvention, uniform pressure conditions will be obtained throughout thesystem. The stable pressure state prevailing in the system, moreover,results in regularly produced amounts of impurities so that thedischarge means will be uniformly charged with dedusted material and thedischarge means such as, e.g., screw conveyors need not be equipped withcomplex control means.

1. A method for dedusting filters for dust-laden waste gases, includingseveral vertically arranged filter elements each having an upper, openend and a lower, closed end, wherein the waste gases flow through thefilter elements from outside and the cleaned waste gases are dischargedthrough the open ends of the same, and wherein for dedustingcompressed-air blasts are blown into the open ends of the filterelements to be dedusted, wherein at least one filter element is eacharranged in a filter module and at least two filter modules are eacharranged in a housing or filter chamber, whereby the filter modules arealternately dedusted with the filter module to be dedusted being set ina substantially flow-free state during dedusting by interrupting, duringdedusting, the clean-gas discharge duct of the filter module to bededusted and effecting the connection of the dedusted filter module to,and the disconnection of the filter module to be dedusted from, the gasflow in a diametrically opposed manner at a change-over of the filtermodules to be dedusted, thus enabling the minimization of any possiblepressure deviations in the filter.
 2. A method according to claim 1,wherein compressed-air blasts at a reservoir or receiver pressure of 0.5to 3 bar, preferably 0.8 to 2 bar, are used for dedusting.
 3. A methodaccording to claim 1, wherein the consumption of the compressed air usedfor dedusting is 0.5 to 4 liters per m² of filter area.
 4. A methodaccording to claim 1, wherein for the dedusting of each filter moduleone compressed-air blast is each used per dedusting cycle for each ofthe filter elements of the filter module.
 5. A method according to claim1, wherein the compressed-air blasts are directed in the direction ofthe filter elements.
 6. A method according to claim 1, wherein thecompressed-air blasts have a duration of less than 300 ms.
 7. A methodaccording to claim 1, wherein the filter module to be dedusted is leftin the substantially flow-free state for a certain period of time afterdedusting.
 8. A device for dedusting filters for dust-laden waste gasesaccording to the method set forth in claim 1, including several filterelements (2) vertically arranged in a filter housing (10) and eachhaving an upper, open end (16) and a lower, closed end (15), at leastone supply duct (9) for the dust-laden waste gases and at least onedischarge duct (11) for the cleaned waste gases as well as a means (4)for blowing compressed-air blasts into the open ends (6) of the filterelements (2), wherein the filter is subdivided into several filtermodules (1) each comprising at least one filter element (2), wherein atleast two filter modules (1) are arranged in a filter housing (10) orfilter chamber, said filter modules (1) being alternately dedusted, andthat each of said filter modules (1) is each associated with a clean-gasdischarge duct (11), in which discharge duct (11) a shutoff means (3) isprovided for the interruption of the clean-gas discharge duct (11), andthat a device (19) for controlling the shutoff means (3) is furtherprovided so as to effect in a diametrically opposed manner, during achange-over of the filter modules to be dedusted, the opening of theshutoff means (3) of the already dedusted filter module (1) and theclosure of the shutoff means (3) of the filter module (3) to bededusted, thus enabling the minimization of any possible pressuredeviations in the filter.
 9. A device according to claim 8, wherein eachof said filter modules (1) is comprised of a chamber forming theclean-gas space (7) and including a connection to the clean-gasdischarge duct (11) as well as a connection to the at least one filterelement (2).
 10. A device according to claim 8 wherein partitionelements (12) are provided between the or some filter modules (1)arranged in the filter housing (10).
 11. A device according to claim 8,wherein at least one nozzle (5) oriented in the direction of the filterelement (2) is arranged above the open end (16) of each filter element(2).
 12. A device according to claim 11, wherein two nozzles (5)oriented in the direction of the filter element (2) are arrangedeccentrically above the open end (16) of each filter element (2).
 13. Adevice according to claim 8, wherein an injector (6) is arranged in theopen end (16) of each filter element (2).
 14. A device according toclaim 12, wherein the at least one nozzle (5) is arranged above the openend (16) of each filter element (2) at a distance from the entry openingof the injector (6).
 15. A device according to claim 8, wherein the atleast one waste-gas supply duct (9) is arranged below the lower ends(15) of the filter elements (2) of the filter modules (1).